1. Field of the Invention
This invention relates to a failsafe weld-free weldless method of joining tubular elements and the connector fitting assembly for carrying out the method. The tubes, inserts and connector fittings used are not size, shape or material specific.
2. Description of the Prior Art
A variety of household and industrial products are fabricated from tubing. Such products include but not limited to, disability aids, metal furniture, serving carts, display units, scaffolding, etc.
Frequently tubular products are brazed or welded to ensure a strong joint. Some of these products include disability aids such as wheelchairs and walkers. Fatigue failures are common in these products at the heat-effected zone of the braze/weld joints. Clamping is another method of joining tubular elements. Tube clamping eliminates some of the problems associated with welded/brazed joints but is more expensive to execute and is not as aesthetically pleasing as utilizing connector fittings. Examples of clamping designs are diclosed U.S. Pat. Nos. 5,253,888, 4,294,561 and 4,039,263. The use of connector fittings greatly increases the structural strength and durability of the tubular joint, provides a failsafe connection and reduces fatigue failure. The connector fitting allows for easy component replacement, modular construction and product customization.
Tube connectors developed for rigidly joining tube sections in such products include compression fittings, as disclosed in U.S. Pat. Nos. 4,921,370 and 3,985,460, that do not positively lock in the inside diameter of one of the tubes and may rotate axially. Other types of tubular connector are described in U.S. Pat. Nos. 4,846,600 and 4,752,979. This design uses an eye bolt type fastener, a notched tube member and a screw to hold the assembly together. With this design, the tubes may distort when the screws are tightened securely and the joint may loosen over time. A design that uses a formed sheet metal nut that is pressed into the inside diameter of a tubular member is disclosed in U.S. Pat. No. 4,810,144. This design relies on spring tension of the teeth of the nut to grip the inside diameter of the tube when a screw is tightened. There is no positive locking, the nut may loosen and/or rotate within the tube. Another disadvantage of all three patented types described above is that the screw attached to the first tube section goes through one or both walls of the tube. This increases stress and deformation of the tube and leaves a screw head showing at the front of the tube. It is possible to crush or deform the tube by the action of substantial forces acting on the tubular walls when the screw is tightened.